Re: Lazy FPU restoration / moving kernel_fpu_end() to context switch

[Andy Lutomirski <luto@kernel.org>]

On Fri, Jun 15, 2018 at 11:50 AM Dave Hansen
<dave.hansen@linux.intel.com> wrote:
>
> On 06/15/2018 11:31 AM, Andy Lutomirski wrote:
> > for (thing) {
> >   kernel_fpu_begin();
> >   encrypt(thing);
> >   kernel_fpu_end();
> > }
>
> Don't forget that the processor has optimizations for this, too.  The
> "modified optimization" will notice that between:
>
>         kernel_fpu_end();   -> XRSTOR
> and
>         kernel_fpu_start(); -> XSAVE(S|OPT)
>
> the processor has not modified the states.  It'll skip doing any writes
> of the state.  Doing what Andy is describing is still way better than
> letting the processor do it, but you should just know up front that this
> may not be as much of a win as you would expect.

Even with the modified optimization, kernel_fpu_end() still needs to
reload the state that was trashed by the kernel FPU use.  If the
kernel is using something like AVX512 state, then kernel_fpu_end()
will transfer an enormous amount of data no matter how clever the CPU
is.  And I think I once measured XSAVEOPT taking a hundred cycles or
so even when RFBM==0, so it's not exactly super fast.